1. Field of the Invention
The present invention relates to the real-time, optical writing and reading of high density information and more particularly to heat-deformation media, methods and apparatus for such writing and reading with light of a single wavelength. As will be apparent to those skilled in the art, the term "light" is used herein in its general sense, i.e., to include electromagnetic radiation within the visible range as well as such radiation outside that range that is useful in practice of the present invention.
2. Description of the Prior Art
In general, high density optical storage media carry information in the form of tracks of small (usually on the order of a micron or less in size), optically-detectable marks formed in the surface of a substrate or in thin material layer(s) deposited on a substrate. Information is recovered (read) by scanning the tracks with a tightly focused spot of light, e.g., from a laser. The recovered information is in the form of a fluctuating electrical signal obtained from a photodetector that senses the read-out light after it has been modulated by the track markings.
There have been a variety of approaches for achieving the general functions outlined above. Although there are others, the most popular storage media format has been a disc with a single spiral track or concentric circular tracks. For convenience, this discussion will refer to the storage media as optical discs, with the understanding that other such record element formats are, in general, equivalent as to utility with the present invention.
With regard to record formation, i.e., recording or writing on optical discs, approaches can be divided as: (1) real-time discs (ones ready for reading immediately after writing) or (2) processed discs (ones requiring further processing after recording before they can be read). Typical of the prior art real-time disc type are heat-deformable elements comprising a substrate bearing a very thin metal or dye layer that is deformed (e.g., displaced or ablated) by the heat generated from an absorbed writing laser beam, which is modulated in intensity according to the signal to be recorded. Typical of the processed disc type are ones formed by: (1) recording exposure of a photosensitive material, such as positive photoresist; (2) chemical development of that material to form a relief pattern and (3) metallization of the relief pattern.
With regard to reading approach, the optical discs can, in one manner, be classified as being of a transmissive or reflective type, depending on whether or not the reading light beam passes completely through the disc or is reflected to a detector on the same side of the disc as the reading light source. The reflective type offers potential simplicity by allowing a single lens to both focus the reading beam on the disc and collect the modulated light returned from the disc.
A more subtle distinction of the disc read-out approaches can be made based on the type of predominating interaction between the focused reading light spot and the recorded marks on the disc. Thus, approaches that obtain a signal based primarily on differences in the electric field amplitude of reading light leaving the marked and unmarked portions of the disc (e.g, because of optical density variations) can be classified as amplitude variation systems. Systems of this type can be written in real-time or be processed. In distinction, approaches which obtain a signal based primarily on differences or transitions in the phase of reading light leaving marked and non-marked portions of a disc can be characterized as phase shift systems. Systems of this type heretofore have not readily been formed in real-time.
In U.S. application Ser. No. 124,381, there is disclosed an improved optical disc which is designed for real-time, heat-deformation recording and which is physically optimized for high sensitivity writing and high contrast, phase shift reading. One significant aspect of that invention is that the recording layer (1) exhibits, at the reflected reading light wavelength, a non-linear relative-phase-shift to recording layer thickness relation characterized by distinct thickness zones of relatively gradual change and relatively rapid change in reflected light phase and (2) has a nominal thickness proximate a transition between such zones. The disclosed disc also is optimized for recording with light of a wavelength different from that used for reading to enhance writing sensitivity, i.e., to reduce requisite writing power. That invention evolved in part from recognition of the utility of the above-described phase shift versus recording layer thickness characteristic in connection with phase interference reading (which is to be distinguished from amplitude variation reading that was traditionally used in real-time write and read systems).
In addition to providing improved writing sensitivity and reading contrast, the approach of the previous application obviated various problems connected with prior art real-time recording systems. For example, in such prior systems it was necessary to reduce the reading light power to avoid re-melting or ablating the recording layer during read-out. Also, when compared to such prior art amplitude systems, the phase interference reading approach enabled high depth of modulation read-out with larger reading spot sizes and less critical focusing and tracking. Although the specific embodiments disclosed in the previous application Ser. No. 124,381 were of discs optimized for writing and reading with different wavelengths, it was noted that certain advantageous features of that invention could be retained in systems using the same wavelength for writing and reading. This would be desirable for some applications where the added cost and complexity of two separate laser devices is not warranted.
An alternative approach for allowing the use of a single writing and reading beam is disclosed in U.S. application Ser. No. 001,519. In accordance with the teaching of that application, the absorption of the recording layer to writing light is reduced, after the recording of information, to a level such that the same light wavelength can be used for reading at a power level that provides useful signal-to-noise levels but without any deleterious re-melting or ablation of the recording layer.